1. Field of the Invention
The field of art to which this invention pertains is the fork carriage for a forklift truck and, more specifically, to an improved fork carriage structure.
2. Description of the Related Art
The structure of a prior art "hook type" fork carriage for a forklift truck is illustrated in U.S. Pat. Nos. 4,124,104 and 3,851,732. This type of carriage is simple, rugged, economical to manufacture and certain of its dimensions have been standardized to permit the interchangeable installation of forks or a wide variety of specialized load handling attachments produced by a number of manufacturers.
Structurally the carriage comprises two vertical, spaced apart sideplates to which are affixed two transverse horizontal crossbars. The carriage is guided, during vertical movement, by rollers which are attached to the sideplates and engage the vertical channels of the forklift truck's upright or mast assembly. The load engaging means, commonly a pair of forks or a specialized load handling attachment, is attached to the fork carriage by hooks, affixed to the load engaging means and engaging the top crossbar. The bottom crossbar may also be engaged by hooks, but these lower hooks do not contribute to supporting the load.
The load forces are transferred from the top crossbar to the sideplates through the T-shaped joints formed by the intersection of the crossbar and the thickness dimension of the sideplates. The load forces include a shear force, due to the weight of the load, and a tension force necessary to restrain the cantilevered forks. These forces, generated by handling loads with cantilevered forks on an unsprung vehicle, are dynamic and cyclic in nature.
While the prior art fork carriage structure is suitable for use in most fork lift trucks, the shear and tension forces exerted by the top crossbar causes the weld, particularly at the top rear corner of the crossbar, to be highly loaded. The length of this top weld and, as a result, its strength is limited by the thickness of the sideplate. Further, the sharp changes in section in the transition from the sideplate to the crossbar result in stress concentration in the welds used to join the crossbar and sideplate and, as a result of the welding process, additional residual stress and stress concentrating inclusions and cracks can be introduced to the weld material. It has not proven to be practical to fully compensate for the increased loading of higher capacity fork lift trucks by increasing the thickness of the sideplate or the distance between the top and bottom crossbar or by the application of conventional methods of welded joint preparation and stress reduction.